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Journal ArticleDOI

A Characterization of African Easterly Waves on 2.5–6-Day and 6–9-Day Time Scales

09 Sep 2013-Journal of Climate (American Meteorological Society)-Vol. 26, Iss: 18, pp 6750-6774
TL;DR: In this paper, a two-dimensional ensemble empirical mode decomposition (2D-EEMD) of the Modern-Era Retrospective Analysis for Research and Applications (MERRA) was applied to analyze the 2.5-6-day and 6-9-day AEW activity over the northern tropical Atlantic.
Abstract: This study shows that the African easterly wave (AEW) activity over the African monsoon region and the northern tropical Atlantic can be divided in two distinct temporal bands with time scales of 2.5–6 and 6–9 days. The results are based on a two-dimensional ensemble empirical mode decomposition (2D-EEMD) of the Modern-Era Retrospective Analysis for Research and Applications (MERRA). The novel result of this investigation is that the 6–9-day waves appear to be located predominantly to the north of the African easterly jet (AEJ), originate at the jet level, and are different in scale and structure from the well-known low-level 2.5–6-day waves that develop baroclinically on the poleward flank of the AEJ. Moreover, they appear to interact with midlatitude eastward-propagating disturbances, with the strongest interaction taking place at the latitudes where the core of the Atlantic high pressure system is located. Composite analyses applied to the mode decomposition indicate that the interaction of the...
Citations
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Journal ArticleDOI
TL;DR: This article examined the association of three global climate teleconnections (ENSO, Indian Ocean Dipole (IOD), and Atlantic Multi-decadal Oscillation (AMO) with changes in terrestrial water storage (TWS) derived from both Modern-Era Retrospective Analysis for Research and Applications (MERRA, 1980-2015) and Gravity Recovery and Climate Experiment (GRACE,2002-2014).
Abstract: There is some evidence of rapid changes in the global atmosphere and hydrological cycle caused by the influence of climate variability In West Africa, such changes impacts directly on water resources leading to incessant extreme hydro-meteorological conditions This study examines the association of three global climate teleconnections (El-Nino Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), and Atlantic Multi-decadal Oscillation (AMO)) with changes in terrestrial water storage (TWS) derived from both Modern-Era Retrospective Analysis for Research and Applications (MERRA, 1980–2015) and Gravity Recovery and Climate Experiment (GRACE,2002-2014) In the Sahel region, positive phase of AMO coincided with above-normal rainfall (wet conditions) and the negative phase with drought conditions, and confirms the observed statistically significant association (𝑟=062) between AMO and the temporal evolutions of standardised precipitation index This relationship corroborates the observed presence of AMO-driven TWS in much of the Sahel region (though considerably weak in some areas) While ENSO appears to be more associated with GRACE-derived TWS over the Volta basin (𝑟=–040), this study also shows a strong presence of AMO and ENSO induced TWS derived from MERRA reanalysis data in the coastal West African countries and most of the regions below latitude 10°N The observed presence of ENSO and AMO driven TWS are noticeable in tropical areas with relatively high annual/bimodal rainfall and strong inter-annual variations in surface water The AMO has a wider footprint and sphere of influence on the region s TWS and suggests the important role of North Atlantic Ocean IOD related TWS also exists in West Africa and its influence on the region s hydrology maybe secondary and somewhat complementary Nonetheless, presumptive evidence from the study indicates that ENSO and AMO are the two major climatic indices more likely to impact on West Africa s TWS

54 citations

Journal ArticleDOI
TL;DR: In this paper, the authors identify the circulation associated with EWDs that propagate toward the Eastern Northeast Brazil (ENEB) and their impact on the rainfall over ENEB during 2006 and 2007 rainy seasons (April-July).
Abstract: This paper aims to identify the circulation associated with Easterly Wave Disturbances (EWDs) that propagate toward the Eastern Northeast Brazil (ENEB) and their impact on the rainfall over ENEB during 2006 and 2007 rainy seasons (April–July). The EWDs identification and trajectory are analyzed using an automatic tracking technique (TracKH). The EWDs circulation patterns and their main features were obtained using the composite technique. To evaluate the TracKH efficiency, a validation was done by comparing the EWDs number tracked against observed cases obtained from an observational analysis. The mean characteristics of EWDs are 5.5-day period, propagation speed of ~9.5 m·s−1, and a 4500 km wavelength. A synoptic analysis shows that between days −2 d and 0 d, the low level winds presented cyclonic relative vorticity and convergence anomalies both in 2006 and 2007. The EWDs signals are strongest at low levels. The EWDs propagation is associated with relative humidity and precipitation positive anomalies and OLR and omega negative anomalies. The EWDs tracks are seen over all ENEB and their lysis occurs between the ENEB and marginally inside the continent. The tracking captured 71% of EWDs in all periods, indicating that an objective analysis is a promising method for EWDs detection.

51 citations

Journal ArticleDOI
01 Mar 2018
TL;DR: The role of global sea surface temperature (SST) anomalies in modulating rainfall in the African region has been widely studied and is now less debated as discussed by the authors, however, their impacts and links to terrestrial water storage (TWS) in general, have not been studied.
Abstract: The role of global sea surface temperature (SST) anomalies in modulating rainfall in the African region has been widely studied and is now less debated. However, their impacts and links to terrestrial water storage (TWS) in general, have not been studied. This study presents the pioneer results of canonical correlation analysis (CCA) of TWS derived from both global reanalysis data (1980–2015) and GRACE (Gravity Recovery and Climate Experiment) (2002–2014) with SST fields. The main issues discussed include, (i) oceanic hot spots that impact on TWS over tropical West Africa (TWA) based on CCA, (ii) long term changes in model and global reanalysis data (soil moisture, TWS, and groundwater) and the influence of climate variability on these hydrological indicators, and (iii) the hydrological characteristics of the Equatorial region of Africa (i.e., the Congo basin) based on GRACE-derived TWS, river discharge, and precipitation. Results of the CCA diagnostics show that El-Nino Southern Oscillation related equatorial Pacific SST fluctuations is a major index of climate variability identified in the main portion of the CCA procedure that indicates a significant association with long term TWS reanalysis data over TWA (r = 0.50, ρ < 0.05). Based on Mann-Kendall's statistics, the study found fairly large long term declines (ρ < 0.05) in TWS and soil moisture (1982 − 2015), mostly over the Congo basin, which coincided with warming of the land surface and the surrounding oceans. Meanwhile, some parts of the Sahel show significant wetting (rainfall, soil moisture, groundwater, and TWS) trends during the same period (1982–2015) and aligns with the ongoing narratives of rainfall recovery in the region. Results of singular spectral analysis and regression confirm that multi-annual changes in the Congo River discharge explained a considerable proportion of variability in GRACE-hydrological signal over the Congo basin (r = 0.86 and R2 = 0.70, ρ < 0.05). Finally, leading orthogonal modes of MERRA and GRACE-TWS over TWA show significant association with global SST anomalies.

49 citations

Journal ArticleDOI
TL;DR: In this article, the authors provide a climatological characterization of wet and dry spells in West Africa, which should serve to assess the ability of climate model to simulate these high impact events.

44 citations

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the relationship between mesoscale convective system (MCS) activity and inter-tropical front (ITF)/dryline dynamics and revealed that over 90% of the Sahel nocturnal rainfall derives from propagating MCSs that have been associated with topography.
Abstract: A convection-permitting regional model simulation for August 2006 and observations are evaluated to better understand the diurnal cycle of precipitation over the Sahel. In particular, reasons for a nocturnal rainfall maximum over parts of the Sahel during the height of the West African monsoon are investigated. A relationship between mesoscale convective system (MCS) activity and inter-tropical front (ITF)/dryline dynamics is revealed. Over 90% of the Sahel nocturnal rainfall derives from propagating MCSs that have been associated with topography in earlier studies. In contrast, in this case study, 70–90% of the nocturnal rainfall over the southern Sahel (11°N–14°N) west of 15°E is associated with MCSs that originate less than 1000 km upstream (to the north and east) in the afternoon, in a region largely devoid of significant orography. This MCS development occurs in association with the Sahel ITF, combined with atmospheric pre-conditioning. Daytime surface heating generates turbulent mixing that promotes planetary boundary layer (PBL) growth accompanied by a low-level reversal in the meridional flow. This enhances wind convergence in the low-level moist layer within 2°–3° of latitude of the equatorward side of the ITF. MCSs tend to form when this vertical mixing extends to the level of free convection and is accompanied by a mid-tropospheric African easterly wave disturbance to the east. This synoptic disturbance enhances the vertical wind shear and atmospheric instability over the genesis location. These results are found to be robust across the region.

44 citations


Cites background from "A Characterization of African Easte..."

  • ...The rest of the disturbances have phase speeds faster than AEWs, but are similar to the estimated phase speeds of organized convection observed over northern Africa (Fink et  al....

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  • ...In turn, AEWs are an important element to creating a favorable environment of enhanced low-level convergence, rising motion, and adequate lowlevel moisture content that localized convective cells can become better organized within, and in some instances, develop into MCSs (Reed et al....

    [...]

  • ...A strong relationship between MCSs and mid-tropospheric AEWs also exists....

    [...]

  • ...That slower disturbance has a zonal phase speed typically associated with 6–9 day AEWs (Diedhiou et al. 1999; Wu et al. 2013)....

    [...]

  • ...1999; Fink and Reiner 2003) that indicate climatologically, AEWs occur both on the equatorward and poleward flanks of the AEJ, which is centered between 12°N−15°N in August....

    [...]

References
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Journal ArticleDOI
TL;DR: In this paper, a new method for analysing nonlinear and nonstationary data has been developed, which is the key part of the method is the empirical mode decomposition method with which any complicated data set can be decoded.
Abstract: A new method for analysing nonlinear and non-stationary data has been developed. The key part of the method is the empirical mode decomposition method with which any complicated data set can be dec...

18,956 citations

Journal ArticleDOI
TL;DR: The Modern-Era Retrospective Analysis for Research and Applications (MERRA) was undertaken by NASA's Global Modeling and Assimilation Office with two primary objectives: to place observations from NASA's Earth Observing System satellites into a climate context and to improve upon the hydrologic cycle represented in earlier generations of reanalyses as mentioned in this paper.
Abstract: The Modern-Era Retrospective Analysis for Research and Applications (MERRA) was undertaken by NASA’s Global Modeling and Assimilation Office with two primary objectives: to place observations from NASA’s Earth Observing System satellites into a climate context and to improve upon the hydrologic cycle represented in earlier generations of reanalyses. Focusing on the satellite era, from 1979 to the present, MERRA has achieved its goals with significant improvements in precipitation and water vapor climatology. Here, a brief overview of the system and some aspects of its performance, including quality assessment diagnostics from innovation and residual statistics, is given.By comparing MERRA with other updated reanalyses [the interim version of the next ECMWF Re-Analysis (ERA-Interim) and the Climate Forecast System Reanalysis (CFSR)], advances made in this new generation of reanalyses, as well as remaining deficiencies, are identified. Although there is little difference between the new reanalyses i...

4,572 citations

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TL;DR: In this paper, Hilbert spectral analysis is proposed as an alternative to wavelet analysis, which provides not only a more precise definition of particular events in time-frequency space, but also more physically meaningful interpretations of the underlying dynamic processes.
Abstract: We survey the newly developed Hilbert spectral analysis method and its applications to Stokes waves, nonlinear wave evolution processes, the spectral form of the random wave field, and turbulence. Our emphasis is on the inadequacy of presently available methods in nonlinear and nonstationary data analysis. Hilbert spectral analysis is here proposed as an alternative. This new method provides not only a more precise definition of particular events in time-frequency space than wavelet analysis, but also more physically meaningful interpretations of the underlying dynamic processes.

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BookDOI
01 Sep 2005
TL;DR: The principle and insufficiency of Hilbert-Huang transform is introduced, several improved strategies are put forward, and some simulations are proceeds some simulations.
Abstract: The Hilbert-Huang Transform (HHT) represents a desperate attempt to break the suffocating hold on the field of data analysis by the twin assumptions of linearity and stationarity. Unlike spectrograms, wavelet analysis, or the Wigner-Ville Distribution, HHT is truly a time-frequency analysis, but it does not require an a priori functional basis and, therefore, the convolution computation of frequency. The method provides a magnifying glass to examine the data, and also offers a different view of data from nonlinear processes, with the results no longer shackled by spurious harmonics — the artifacts of imposing a linearity property on a nonlinear system or of limiting by the uncertainty principle, and a consequence of Fourier transform pairs in data analysis. This is the first HHT book containing papers covering a wide variety of interests. The chapters are divided into mathematical aspects and applications, with the applications further grouped into geophysics, structural safety and visualization.

847 citations

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the origin and structure of easterly waves that form in the lower troposphere of North Africa and have a periodicity of 3-5 days.
Abstract: This study investigates the origin and structure of easterly waves that form in the lower troposphere of North Africa and have a periodicity of 3–5 days,. From June to early October these waves propagate across the Atlantic and occasionally reach the eastern Pacific. Although only a few of these disturbances actually intensify after reaching the Atlantic, they account for approximately half of the tropical cyclones that form in the Atlantic. Spectral analysis of five years of upper air data shows that African waves produce a spectral peak of the meridional wind at periods of 3–5 days with a maximum amplitude of 1–2 m sec−1 near 700 mb. These waves normally originate between Khartoum (32E) and Ft. Lamy (I5E) and affect a greater depth of the atmosphere as they propagate westward. Wind statistics at stations flanking the mountains in Ethiopia indicate that airflow over these mountains is not the cause of the easterly waves. This study shows that the African waves are directly related to the mid-tro...

614 citations